1. Field of the Invention
This invention relates to a solid ion conductive polymer electrolyte utilizable as an electrochemical material particularly for rechargeable batteries (secondary batteries), capacitors and the like.
2. Description of the Background Art
As the electrolytes of rechargeable batteries, capacitors etc. there have mainly been used liquid substances such as water, propylene carbonate, tetrahydrofuran and the like.
Since a liquid electrolyte is apt to leak, however, a hermetically sealed container has to be used to ensure its long-term stability.
Because of this, electrical and electronic devices using liquid electrolytes are heavy and require complex manufacturing processes.
In contrast, electrolytes consisting of solid ion-conductive material involve almost no possibility of leakage, simplify manufacture and enable reduction of product weight. Owing to these advantages, they are being vigorously researched.
Solid ion conductive electrolytes can be divided into inorganic and organic material types. Organic ion conductive solid electrolytes are superior to inorganic solid ion conductive electrolytes in the points of weight, formability and flexibility.
Organic solid ion conductive electrolytes are generally formed of a matrix polymer and an ion conductive metallic salt which is a low molecular weight compound.
The matrix polymer is the most important constituent of an organic solid ion conductive electrolyte because it is responsible both for solidifying the electrolyte and for serving as a solvent for dissolving the ion conductive metallic salt.
In 1978, M. B. Armand et al., working at the University of Grenoble in France, discovered that lithium perchlorate dissolves in ethylene oxide and reported that this system exhibits ionic conductivity of 10.sup.-7 S/cm. Since then, similar research has been conducted regarding analogous polymers, including polypropylene oxide, polyethyleneimine, polyurethane, polyester and a wide range of other polymeric substances.
Application of organic polymers to solid electrolytes for rechargeable batteries is being pushed forward for taking advantage of their various merits, which include excellent film formability, flexibility and high energy characteristics when used in batteries.
Polyethylene oxide, which has been most thoroughly researched, is a polymer with high capacity for dissolving ion conductive metallic salts. However, it is a semicrystalline polymer, and when a large amount of metallic salt is dissolved therein, it forms a quasi-crosslinked structure that increases its crystallinity even further. As a result, the conductivity obtained is considerably lower than might be expected.
Ionic conductors dissolved in a matrix of linear polyether polymer such as polyethylene oxide migrate in the amorphous region above the glass transition temperature of the polymer matrix owing to local segment motion of the polymer chain.
Since the cations, which are responsible for the ionic conductivity, interact strongly with the polymer chain, their mobility is markedly affected by local segment motion of the polymer chain.
From the aspect of ionic conductor mobility, therefore, it is not wise to choose a linear polymer as the matrix polymer for a solid ion conductive polymer electrolyte.
Reported solid ion conductive polymer electrolytes consisting solely of linear polymers, such as polyethylene oxide, polypropylene oxide and polyethyleneimine, have room-temperature conductivities of 10.sup.-7 S/cm or, at the very highest, 10.sup.-6 S/cm.
To secure high ionic conductivity at room temperature, it is important to ensure the presence of many amorphous regions in which the ionic conductors can migrate and to use a polymer design which lowers the glass transition temperature of the polymer.
A method of introducing a branched structure into polyethylene oxide attempted for this purpose led to the synthesis of a polyethylene oxide derivative which exhibited high conductivity (about 10.sup.-4 S/cm at room temperature) as a solid ion conductive polymer electrolyte (Naoya Ogata et al., Sen'i Gakkaishi (Journal of the Society of Fiber Science and Technology, Japan) Vol 46, No 2, p52-57, 1990). Owing to the complexity of the polymer synthesis method, however, the method has not been commercialized.
Another method proposed for securing ionic conductivity is that of imparting a three-dimensional network structure to a matrix polymer so as to prevent its crystallization.
Such a method is taught, for example, by Japanese Patent Public Disclosures Hei 4-112460 and 5-36483, which obtain a solid ion conductive polymer electrolyte by crosslinking and curing a polyoxyalkylene derivative of glycerin with polyisocyanate compound.
With this method, however, still unsolved problems arise:
Isocyanate reacts easily with moisture and is therefore difficult to manage from the points of storage and reactivity.
The urethane crosslinking reaction between the polyoxyalkylene derivative of glycerin and the polyisocyanate compound is affected by the ion conductive metallic salt and solvent components. As a result, the reactivity may be reduced or the reaction be accelerated. Because of this, the method of synthesizing the polymer matrix first and then impregnating it with the ion conductive metallic salt together with an appropriate solvent (the impregnation method) is generally used, despite its poor industrial productivity.
General-purpose aromatic isocyanate is susceptible to electrochemical degradation, while the reactivity of aliphatic isocyanate is low.
Formation into film requires a long period of reaction under heating.
Another example of using a polymer with a three-dimensional network structure for the polymer matrix is disclosed in Japanese Patent Public Disclosure Hei 5-25353, which teaches a method of polymerizing an acrylic or metacrylic monomer including a polyoxyalkylene component. Since the solubility of the ion conductive metallic salt in the monomer is low, however, the method is disadvantageous from the points that it requires addition of a third component such as vinylene carbonate and that the polymer obtained is low in physical strength.